A digital watermarking technique of embedding specific information in an image or the like is known in the related art. When this technique is used for printed matters, information can be embedded at a level at which the information cannot be visually recognized by a person, and thus artistic effects of printed matter are not damaged. Information embedded in printed matter can be read by a reading device.
As printed matter using the digital watermarking technique, printed matter which includes a plurality of line images having a color element having high brightness and a color element having low brightness, in which the line image including the color element having high brightness has an area from which concentric lines set to extract frequency components corresponding to information to be embedded are disposed and the line image including the color element having low brightness has an area formed by an image group including frequency components other than the frequency components of the concentric lines and forming a shape different from the concentric lines is disclosed (for example, Patent Document 1).
In the printed matter disclosed in Patent Document 1, information is embedded in the line image formed by the color element having high brightness. A shape obtained by frequency-converting the line image of the color element having low brightness and a shape obtained by frequency-converting the line image of the color element having high brightness are different from each other. Since the frequency-converted shapes are different from each other, it is possible to estimate pre-frequency-conversion shapes of the line images.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2013-70220